CHEMICAL AND STRUCTURAL ANALYSIS OF THE LOWER CRESCENT FORMATION AT MT. TEBO AND LAKE CUSHMAN, OLYMPIC PENINSULA, WASHINGTON: EVIDENCE FOR TERTIARY RIFTING

The Crescent formation (Cf) is an unusually thick sequence of subaqueous and subaerial basalts interbedded with sedimentary units which formed 62-45 Ma. The Cf and the Olympic Core terrane compose the bulk of the Olympic Peninsula in northwestern Washington. The Cf is comprised of three members: the Blue Mountain unit (BMu), the Lower Crescent member (LC), and the Upper Crescent member (UC). The BMu is a continentally-derived fan which underlies and is interbedded with the LC. The LC is primarily composed of subaqueous pillow basalts with a Zr ratio of less than 125 ppm, while the UC grades from subaqueous pillow basalts to subaerial massive and jointed basalts which have a Zr ratio of greater than 125 ppm. We worked in the LC at two study sites, one at Mt. Tebo in the Skokomish Wilderness Area and the other at Lake Cushman about 10 miles west of Hoodsport on Hwy 119, to expand upon the existing body of work on the Cf. We mapped and collected 23 samples for petrographic and chemical (ICP-ES) analysis. At both sites, the samples were enriched in calcium, and petrographic analysis showed amygdaloidal weathering with secondary mineral replacement. Chemical analysis, through the use of discrimination diagrams, indicates formation in a rift zone. Mt. Tebo had one lava flow containing unusually high amounts of iron and titanium, as well as abundant foraminifera in the sedimentary samples. Lake Cushman samples had undergone low grade metamorphism and faulting was abundant. A set of conjugate faults which may have caused the lake's shape was also found. The Cf formation is of interest as its origin is controversial. Our work indicates that the Cf was formed in a rift setting consistent with that described by Babcock and others (1992). Chemical signatures display enriched MORB characteristics and OIB markers. These two characteristics along with the overriding plate motions for the region are consistent with the subduction of a mid-oceanic spreading ridge beneath the North American continental margin.